The present invention generally relates to increased boiling heat transfer. More particularly, this invention relates to a free-particle technique wherein free particles are located on a superheated surface submerged in a liquid during boiling in order to increase boiling heat transfer.
The heat dissipation efficiency of phase-change processes for cooling high-performance microprocessors and thermal management of industrial engines, reactors, and plants, has motivated development of numerous techniques for pool boiling heat transfer enhancement. As the performance and density of modern electronics and electromechanical systems rapidly increase and phase-change cooling becomes more prevalent, concepts for facilitating bubble nucleation at reduced superheat temperatures and intensifying the nucleate boiling process have been suggested for heat transfer enhancement. Boiling heat transfer enhancement techniques usually fall into three categories: integral surface roughness, surface coatings, and attached nucleation promoters. The typical enhancement mechanisms of surface treatment techniques for boiling heat transfer enhancement provide preferential nucleation sites on the heated surface or alter the physical properties of the working fluid.
There is long-standing acknowledgment that surface roughening provides improvement in boiling heat transfer. However, reliance on this enhancement has been considered commercially intractable due to aging effects and difficulty in achieving repeatable/predictable performance. Alternatively, since the contact angle of a liquid on a heated surface affects bubble nucleation, researchers have investigated nonwetting surface coatings, such as paraffin and Teflon. Since nonwetting coatings are able to provide nucleation sites at a lower surface superheat compared to wetting surfaces, nucleate boiling heat transfer can be improved at relatively lower heat fluxes. However, large contact angles limit nucleation site density and vapor blanketing of the surface occurs at a lower superheat. Nonwetting layers formed of low thermal conductivity materials also undesirably increase the surface thermal resistance. Patterning surfaces with alternating wetting and nonwetting areas to bypass this tradeoff inherent to homogeneous non-wetting coatings have also been proposed and investigated.
Significant enhancements of pool boiling have been realized by various types of attached promoters, such as porous particle layers, wire meshes, and pin-fin structures. These promoters are usually thermally conductive and directly attached to the superheated surface, for example, through sintering. Compared to smooth surfaces, changes in local surface topography due to the presence of the attached promoters facilitates bubble nucleation at lower wall superheats, and results in more effective heat transfer from the surface to the working fluid. In order to increase both the nucleate boiling heat transfer coefficient and critical heat flux (CHF), the microscale structures in these studies were also designed to reduce liquid-vapor counterflow resistance, that is, the flow resistance to vapor escaping from a surface and to liquid returning to the gaps and cavities where it was vaporized. Despite nucleate boiling heat transfer improvements, the cumbersome processes required for fabrication and attachment of these microstructures to a surface is often considered a drawback. Therefore, simpler methods have been proposed for spraying/painting a mixture of metal particles, binder, and carrier on a target surface to form particle porous layers attached to the surface. However, deterioration of boiling heat transfer has been reported as a result of an increase in thermal resistance between the heated surface, particles, and working fluid due to the use of binders that exhibit relatively low thermal conductivity.
An additional enhancement technique not described above involves the use of fluid additives. In particular, use of nanoscale particles as additives in nanofluids for boiling heat transfer enhancement has been studied, but the effects on boiling heat transfer have been subject to dispute. A range of observations, such as mild improvement, mild deterioration, and negligible impact on boiling heat transfer, have been reported for nanofluids. One commonality is improvement in CHF reported by several studies on nanofluids, such as S. M. You, J. H. Kim, K. H. Kim, Effect of nanoparticles on critical heat flux of water in pool boiling heat transfer, Applied Physics Letters 83 (16) (2003) 3374-3376, and S. J. Kim, I. C. Bang, J. Buongiorno, L. W. Hu, Surface wettability change during pool boiling of nanofluids and its effect on critical heat flux, International Journal of Heat and Mass Transfer 50 (2007) 4105-4116, whose results are primarily attributed to deposition of thin particle layers on a heated surface during nucleate boiling process. The deposited particle layers increase wettability of the surface and decrease the contact angle between the surface and the working fluid, thus resulting in CHF enhancement.
In view of the above, there is an ongoing desire for improved boiling heat transfer methods that are capable of increasing the nucleate boiling heat transfer coefficient and CHF of a working fluid.